This invention relates to well pumping units for reciprocation of a downhole pump and, particularly, to an improvement in the design of such units for decreasing the cost of operation and manufacture of them.
The conventional well pumping unit includes a walking beam rotatably mounted on a saddle bearing located at the upper end of a samson post. An arc is attached to the end of the walking beam. The arc is adapted for connection to a string of sucker rods extending downwardly in a well to a downhole pump. A motor-driven rotary crank oscillates the walking beam through a pitman rotatably connected at one end to the walking beam by a tail bearing and at the opposite end to the crank by a wrist pin. A counterweight attached to the crank assists the motor in oscillating the beam on the upstroke of the well string. The conventional units often have a pair of cranks, each having a counterweight attached, and a pitman connected to each crank and the walking beam.
It is known that the peak torque load on the motor can be reduced by modification of the dimensions and placement of the various parts of the pumping unit. By virtue of such modifications, it is possible to slow the upstroke of the well string and decrease the peak torque load required to be exerted by the motor on the crank. The slower upstroke is evidenced by the fact that it occurs on more than a 180 degree portion of rotation of the crank, whereas the resultant faster downstroke occurs on a commensurately less than 180 degree portion of crank rotation. An example of such a modified pumping unit is disclosed in U.S. Pat. No. 3,406,581, Eyler, et al. The crank in the pumping unit of this reference is located so that the entire circle of rotation of the wrist pin lies to the rear and out from under the area directly beneath the operating arc of the tail bearing. The slower upstroke which results is illustrated by the fact that it occurs over a 210 degree portion of the arc of rotation of the crank, compared to a 150 degree portion of the arc for the downstroke.
It is also well known in the art to use an angularly offset wrist pin connection between the pitman and the crank so that the maximum assist from the counterweight to the motor is obtained when the well load is at its highest level. To accomplish this, the counterweight should be located so that its center of gravity is at either the 3 o'clock or 9 o'clock position on the circle of rotation of the crank when the well string is at the midpoint of its upstroke. The previously mentioned change in the circle of rotation of the wrist pin as disclosed in Eyler, et al. requires a change in the phase or timing of the maximum force exerted by the counterweights with respect to the well load. An angularly offset wrist pin can be used as disclosed in Eyler, et al. to obtain most efficient utilization of the motor torque and to decrease the peak torque load required from the motor. A significant disadvantage of the well pumping units disclosed in the Eyler, et al. reference is that the distance between the motor and the samson post is increased from that of prior units. Because of this increased distance, a longer bottom frame is required increasing the cost of the unit. Also, a longer concrete base is needed for support of the frame, increasing the cost of installation. A second disadvantage results from the use of an angularly offset wrist pin on these units. The casting for the two crank arms are not interchangeable because the wrist pin of one crank arm would be in the wrong location for the other. Thus, separate castings must be provided for each crank arm on a twin crank arm unit of this type.
Therefore, it is a primary object of this invention to overcome the aforementioned disadvantages of prior art pumping units, while retaining the advantages of a slower upstroke and proper positioning of the counterweight so as to obtain the maximum assist by the counterweight to the motor when the well string is at the midpoint position of its upstroke.